If your original image wasn't opened as a 16-bit file, you've essentially thrown away much of the pixel data that your camera initially recorded. Changing back to 16-bit after you've processed the Raw file won't bring that information back. If you want to squeeze out every last bit of your camera's performance you'll need to go back to the original file and open it as a 16-bit image.

I've never tried switching from 8- to 16-bit after opening a file, but I'd be surprised if it affected your prior adjustments.

Like Clockwise said. Once the data's gone you can't get it back. Changing up to 16 bits from 8 bit will just fill out the extra bits with useless "0"'s so you're just adding to the file size and acheiving nothing else. If you've started in 8 bit just leave it there.

However, although working in 16 bit is ideal, often the differences are subtle, in the final product so don't stress too much about it. It's likely reworking the file will bring only small improvements that may not even be noticable. If you do start to see posterisation in colour bands (say large sections of sky) of sudden and severe clipping then maybe go back and redo the file. Otherwise don't worry about it.

It has to be done when you have the Raw file open in ACR, before you process the file and open it in Photoshop or Lightroom. Click on the blue line of text under the main image window in ACR, and a dialog will pop up where you can choose your processing settings. At least that's how it's done in ACR 6, not sure about the new version but I suspect it's a similar process.

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